1. Technical Field
The present invention relates, generally, to a state-of-charge indicator which exhibits the dynamic capacity of a battery and, especially, a lead-acid storage battery. The present invention relates more particularly to a state-of-charge indicator which is generally self normalizing with respect to variables such as temperature and age of the battery. This invention involves both the indicator, per se, and its method of implementation.
2. Description of the Background Art
The art of lead-acid storage batteries is highly refined; these devices have been utilized commercially for about 125 years. Since its introduction, the lead-acid storage battery has distinguished itself as a highly efficient and reliable electrochemical energy source relying on the double sulfate reaction, characterized by an energy efficiency in excess of 80% with coulombic efficiencies as high as about 95%. Lead-acid storage batteries are also relatively insensitive to debilitating temperature effects over a broad practical range of about -40.degree. to about 160.degree. F., lending them to a broad range of utility. Accordingly, the commercial adaptability of lead-acid storage battery technology continues to burgeon.
Transportation is one example of a commercially intriguing use for lead-acid storage batteries. Batteries have been used for small or personal vehicular movement for quite some time. For example, batteries have been used effectively for golf carts, wheelchairs, or similarly small-scaled transportation devices. Great interest has been peaked in the adaptability of storage batteries to provide adequate locomotion for larger vehicles. The so-called "electric car" relies heavily on the regenerative ability of lead-acid storage batteries to provide adequate power for transportation over reasonable distances, such as 50 or more miles, without the need for recharging. Along these lines, it is of obvious importance to a user of such a vehicle that he has the ability to complete his travels at a destination having the ability to recharge or replace the batteries responsible for motive power. Currently, because of the rarity of electric vehicles for general transportation, facilities do not exist as a general matter and travel must be carefully scheduled and monitored. In turn, these needs compel a reasonable ability to monitor the state-of-charge of the energy source much like an internal combustion engine is accompanied by a fuel gauge to indicate the quantity remaining for continuing travel.
The need for a reliable state-of-charge indicator has not gone unnoticed in the prior art. A wide variety of proposals have surfaced to monitor the state-of-charge of a battery or battery system on a real-time basis. The following systems are representative.
U.S. Pat. No. 4,423,378 discloses an automotive battery test apparatus for examining the condition of a storage battery used as a "SLI" (starting, lighting and ignition) battery for an internal combustion engine. The system disclosed in the '378 patent tests the condition of the storage battery and provides an indication under varying conditions. More specifically, the battery testing device measures battery voltage under (a) open circuit conditions; and (b) while the battery is subjected to a predetermined AC load and a predetermined DC load. The temperature of the battery is monitored as well. A microprocessor utilizes the open circuit potential, the measured potential under both DC and AC loading, and the temperature to ascertain the characteristics of the battery. For example, internal resistance is determined and, should it be found to be excessive (e.g., greater than 20 millions), the battery is deemed defective. Open circuit voltage, internal resistance and temperature provide inputs for calculating the estimated power at a fully charged state. The apparatus then discharges the battery through a reference load for about 15 seconds at constant load and measures a 15 second battery voltage; this voltage is then compared to a similar voltage of a battery at about 75% state-of-charge under the same condition. If the measured voltage is higher than that from the library values maintained on the computer, the battery condition is deemed good. Thus, performance benchmarks are ascertained with a view toward determining battery capability.
U.S. Pat. No. 4,433,295 also concerns an apparatus for determining the general state-of-charge of a battery. This approach, however, requires the battery to be taken from its circuit and associated with a calibrated resistor to determine state-of-charge. The process requires the battery to be subjected to two loads, one corresponding to a minimum current consumption level and the other a maximum consumption level or load. The process includes monitoring the load condition between the minimum and maximum load possibilities, periodically connecting the battery to a reference load when the minimum load is applied to the battery sampling the voltage across the reference load and comparing the sample voltage to an array of predetermined levels, each corresponding to a different state-of-charge. The comparison thereby yields an indication of the current conditions of the battery.
U.S. Pat. No. 4,394,741 concerns an interesting approach to determining state-of-charge based on the integration of current. During the first portion of the discharge, the device evaluates the state of charge by integrating the current after compensating it for the rate of discharge. Later in the discharge the state of charge of the battery is determined from the lowest subpack voltage corrected for polarization. Battery polarization is used to calculate a corrected battery voltage which is used to terminate discharge at an appropriate time.
U.S. Pat. No. 4,423,379 discloses a "go/no/go" battery tester which pulls a high current to estimate internal resistance and to understand the condition of the battery under test. The system disclosed in the '379 patent is microprocessor controlled, as are several of those discussed above, allowing rapid evaluation of the battery under test.